1. Field of the Invention
The present invention relates to a sheet switch module which is, for example, installed in an operation panel used for various electronic instruments and has an illumination function.
2. Description of Related Art
Most conventional operation panels installed in various electronic instruments such as mobile phones, mobile information terminals or the like have been structured using key switches incorporating an illumination function. Various illumination functions have been proposed, including a structure in which light sources such as light emitting diodes (LEDs) are disposed corresponding to individual pressing parts (key tops) which constitute the key switches, or alternatively, a structure making use of a light-guiding plate, whereby each of the key tops can be efficiently illuminated by a minimum number of LEDs (for reference, see Japanese Patent Laid-Open No. 2004-69751, FIG. 9).
FIG. 7 illustrates a structural example of a key switch 1 using a light-guiding plate.
The key switch 1 includes a circuit board 2 provided with a plurality of fixed contacts 3 and wiring patterns (not shown), a movable contact 4 which comprises a tact spring and is disposed to cover each of the fixed contacts 3, a light-guiding plate 5 disposed above the circuit board 2, LEDs 6 to illuminate a side surface of the light-guiding plate 5, and a key top 7 which has a lower end portion disposed to pass through the light-guiding plate 5 and is configured to press each movable contact 4.
In the key switch 1, all the key tops 7 can be illuminated by guiding light from the LEDs 6 to the lower end portion of each of the key tops 7 passing through the light-guiding plate 5.
To deal with the recent trend towards thinning of electronic devices, there has also been proposed an even thinner key switch having an illumination function (for reference, see Japanese Patent Laid-Open No. 2004-69751, FIG. 6).
FIG. 8 illustrates a structure of a key switch 11 disclosed in Japanese Patent Laid-Open No. 2004-69751.
The key switch 11 includes a sheet switch part 18 having a circuit board 12 provided with fixed contacts 13 and movable contacts 14 disposed to cover the fixed contacts 13, a light-guiding plate 15 covering above the sheet switch part 18, LEDs 16 disposed to face a side surface of the light-guiding plate 15, and key tops 17 disposed above the light-guiding plate 15 to face the movable contacts 14. In the key switch 11, the entire light-guiding plate 15 is illuminated by directing light emitted from the LEDs 16 to the side surface of the light-guiding plate 15, and the key tops 17 are illuminated from the under side by the light emitted from the light-guiding plate 15.
However, in key switches having the above-mentioned kind of structure in which the LEDs are disposed in the vicinity of the key tops, the number of LEDs must be increased as the number of key tops increases.
This results in problems of increased power consumption and a key switch being larger.
In the key switch 1 as shown in FIG. 7, because the light is guided toward the key tops by the light-guiding plate 5, sufficient light can be supplied by a small number of LEDs, but it is necessary for the light-guiding plate 5 to have a thickness greater than a certain minimum level in order to efficiently guide light emitted from the LEDs 6 toward the key tops without leakage of light. However, because the increment in thickness of the light-guiding plate causes an increase in thickness of the key switch, the problem arises that the key switch cannot be thinned.
On the other hand, in the key switch 11 as shown in FIG. 8, because light emitted from the LEDs 16 enters one end of the light-guiding plate 15 for the light-guiding plate 15 being illuminated and is guided through the light-guiding plate 15 to the entirety of the key tops 17, the problem arises that brightness of the key tops 17 distant from the LEDs 16 is reduced and consequently variations in brightness of the key tops 17 become highly visible. Also, because the light-guiding plate 15 is merely placed on the sheet switch part 18, a gap is easily generated between the light-guiding plate 15 and the sheet switch part 18. In addition, a gap is easily generated between the light-guiding plate 15 and each of the LEDs 16. When a gap arises in a circumferential part of the light-guiding plate 15, light leaks from the gap as a result and the problem arises that an insufficient amount of light is guided into the key tops 17 and variations in brightness occur. Therefore, it is difficult to achieve an illumination effect of the key tops 17 only being intensively lighted as a conventional device.